The invention concerns an improvement in apparatus for high pressure-charging an internal combustion engine, the pressure drop of the engine exhaust gases being divided into two stages and used to compress the combustion air in two stages.
Exhaust-gas turbochargers are today used almost exclusively for pressure-charging internal combustion engines in order to raise the mean effective pressure. Gas-dynamic pressure-wave machines for this purpose are also known, distinguishing features of these being particularly their rapid response to load changes and the fact that they improve the flexibility of the engine, for which reasons they are employed mainly with vehicle diesel engine (Motortechnische Zeitschrift No. 31 (1970) 1).
A two-stage method of pressure-charging using exhaust-gas turbochargers has also been tried for very high output engines (Swiss Patent No. 371633), the pressure drop of the expanding exhaust gases from the engine being divided between a high-pressure turbine and a low-pressure turbine. In the same manner, the compression of the combustion air took place in a low-pressure stage and a high-pressure stage. Dividing the pressure drop in this way is necessary if the charging-pressure conditions attainable in one stage of the turbocharger are no longer adequate, or the volume throughput range of the compressor at the requisite high pressure ratios is so narrow that it no longer meets the demands made of the engine. With two-stage pressure charging, each stage operates at higher efficiency with a correspondingly smaller pressure ratio over a wider range of throughput.
The use of two exhaust-gas turbochargers in series for the vehicle engine, however, seriously impairs the acceleration capability of the engine/turbocharger system and also the flexibility of the engine. Because the turbochargers are "coupled" with the engine only by way of the exhaust gas, they follow engine load changes only after a certain delay, the length of which depends mainly on the mass inertia of the turbocharger rotor, the method of pressure-charging itself, and on the operating ranges which have to be passed through. It is true that with two-stage pressure-charging the smaller high-pressure turbocharger can be operated on the impulse principle and therefore reacts comparatively quickly to load changes. But the larger low-pressure turbocharger, which has a much greater mass inertia and, moreover, is operated on the constant-pressure principle, follows only very slowly. The result is heavy smoke emission on accelerating, and long response times by the engine. Owing to its large inertia, this system of two-stage pressure-charging is not applicable to vehicle engines which are required to respond very quickly to load changes in order to adapt speedily to changing road and traffic circumstances. This approach thus offers no access to very high mean pressures.
Two-stage pressure-charging with pressure-wave machines alone is also not possible, because with these machines the expansion part and the compression part are not independent of each other. Owing to the direct contact between the expanding gas and the gas being compressed, the pressure levels of these cannot be chosen at will, as otherwise the machine will not operate in the required manner. As far as two-stage pressure-charging with pressure-wave machines is concerned, this fact creates difficulties especially at the lower pressure stage of the high-pressure machine.